1. Field of the Invention
The present invention relates to a beam splitting optical system for an auto-focus sensor, which can be utilized in an optical instrument, and in particular with a surveying instrument.
2. Description of the Related Art
In a conventional automatic focusing device for a surveying instrument having a collimator telescope (such as a total station), a light path of a focus detection optical system is split from a light path of a collimating optical system by a beam splitting optical system to detect the focus state on a surface (referred to as a reference focusing surface) which is optically equivalent to the focusing surface of the collimating optical system by means of a phase difference detection type auto-focus sensor module having a pair of CCD sensors, in order to calculate the amount of defocus of a focusing lens. Consequently, the focusing lens is moved to an in-focus position in accordance with the defocus amount to complete an automatic focusing (AF) operation. The principle of the AF function in which the phase difference is detected is known in the art, and is used in an AF single lens reflex camera.
In a conventional beam splitter system for an auto-focus sensor, the beam splitter and the auto-focus sensor are arranged so that the beams of light received by the pair of CCD sensors are made incident upon a coated splitter surface of the beam splitting optical system at different incident angles. In this arrangement, since the transmittance of the splitter surface which is made of a multi-layered dielectric film varies depending on the incident angle, there is a difference in the level between the quantities of light received by the pair of CCD sensors. To prevent a level difference occurring, it is necessary to arrange the optical elements so that the incident angles of the beams incident upon the split surface are identical. This reduces the freedom of design of the layout of the optical elements, hindering any possible enhancement in operational efficiency, hindering miniaturization and reduces the weight of the optical system.
The difference in the quantity of light caused due to the above-mentioned arrangement is usually corrected using a correction coefficient in the determination of the focus during the automatic focusing operation. However, if the difference is large or if electric noise is produced in the auto-focus sensor itself, the noise is increased according to the correction coefficient, thus resulting in a failure to perform a precise auto-focusing operation. This tends to occur when the quantity of light is small, for example, at dusk.
It is an object of the present invention to eliminate the above-mentioned drawbacks of the prior art, by reducing the difference in the quantity of light received by a pair of CCD sensors within the auto-focus sensor to thereby reduce the restriction of the arrangement of the components in the beam splitting optical system.
In order to achieve the above-mentioned aim, there is provided a beam splitting optical system for an automatic focusing apparatus including: a telescopic system having an objective optical system and a viewing optical system; a beam splitter which splits object-carrying light transmitted through the objective optical system from the telescopic optical system via a splitter surface; and a focus detection optical system having a pair of light receiving elements which respectively receive an object-carrying light beam split by and transmitted through the splitter surface of the beam splitter, the focus detection optical system being arranged so that the object-carrying light beam to be respectively received by the respective light receiving elements is incident upon the splitter surface at different incident angles The splitter surface of the beam splitter is coated with a thin metal coating. As the angle-dependency of a metal coating is low, even if the incident angle of the object-carrying light beam on the splitter surface differs, the difference in the amount of light to be received by the AF sensor is minimal.
According to another aspect of the present invention, there is provided a beam splitting optical system for an automatic focusing apparatus including: a telescopic system having an objective optical system and a viewing optical system through which an object image formed on a predetermined focusing surface via the objective optical system can be viewed; a beam splitter which splits object-carrying light transmitted through the objective optical system from the telescopic optical system via a splitter surface; and an auto-focus sensor module which detects a focus state on a reference focusing surface which is optically equivalent to the predetermined focusing surface, on the light path of the object-carrying light split by the beam splitter; the auto-focus sensor module being provided with a pair of optical sensors, the auto-focus sensor module and the beam splitter being arranged so that an object-carrying light beam to be respectively received by the pair of optical sensors is incident upon the splitter surface of the beam splitter at different incident angles. The splitter surface of the beam splitter is coated with a thin metal coating.
Preferably, the thin metal coating includes aluminum, silver, gold, nickel or chrome. The thickness of the metal coating is determined according to the desired amount of the transmittance and the type of the metal used.
Preferably, the splitter surface is defined by one of a plurality of reflection surfaces of an optical element which constitutes an image erecting optical system provided in the telescopic optical system.
Preferably, the optical element which constitutes an image erecting optical system is a Porro prism.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 10-126232 (filed on May 8, 1998) which is expressly incorporated herein by reference in its entirety.